The present invention relates, in general, to plastic caps for containers, and more particularly to plastic vacuum sealing caps.
Particularly in food packing applications, but in other application as well where material in a container is subject to spoilage or degeneration in the presence of air, it is the practice in many cases to produce a relatively strong vacuum within the container for such material and to seal the container by means of a suitable closure such as a cap or cover to prevent air from entering the container. To obtain and to maintain a suitable vacuum over a period of time sufficient to give reasonable shelf life to the contents of the container, it is necessary to provide a secure and reliable sealing of the cover-to-container interface. In the past, a wide variety of cover designs and configurations have been provided for containers, with many being in the form of metal caps. However, for economic reasons and because there have been some indications that metal caps may adversely affect some foods, there has been an effort to develop an effective plastic cap for vacuum sealing containers, and in particular for sealing glass bottles and jars.
Over the years, many attempts have been made to develop satisfactory plastic caps for this purpose, but numerous problems have been encountered. For example, it has been found that many plastics deteriorate over a period of time, causing cracking and breaking of the cap and resulting in a poor shelf life for the packaged product. Even if the material did not crack or break, it would often gradually deform over a period of time allowing leakage around the edges of the cap and consequent spoilage of the contents. The problems of deterioration and deformation are being overcome to a large extent by the development of new plastic materials, and interest in such caps has been renewed, for it has long been known that there is a real economic advantage in plastic caps because of the ability to produce them in high quantities at reasonable cost.
Any immediate change to plastic caps has, however, been seriously impeded by the fact that heretofore there has not been a suitable structural design for a cap that would enable a manufacturer to take advantage of the apparent superiority of this material. The difficulty was that in all of the prior designs, a very exact fit between the container edge and the cap was required if the cap was to produce an effective seal, but the manufacturer was faced, in the typical manufacturing environment, with the problem of variations in the wall thickness of glass bottles and jars. With conventional molding techniques for the production of glass containers, wide variations occur in the thickness of the neck wall, even between containers made from the same mold. With prior plastic cap designs, the inside of the bottle or jar neck wall had to be precisely molded to insure that there were substantially no variations from bottle to bottle in order to create a proper seal between the bottle and the cap intended for it. In addition to the problem of variations in wall thickness from bottle to bottle, numerous irregularities occur along the mouth edges of such containers, either as a result of the molding process or because of chipping of the glass in handling the container, the latter problem being particularly serious when glass containers are returned for reuse. To eliminate such variations in the container to accommodate plastic caps would have required changes in glass molding techniques that would have greatly increased the costs of such containers, and would have put an end to recycling used containers.